The present invention relates generally to devices and methods for performing neural stimulation and more specifically to devices and methods that employ ultrasound to achieve neural stimulation.
Nerves in higher biological organisms are bundles of long, excitable cells that can extend to meter-order lengths. Cells are referred to as excitable when they are capable of responding to various electric, chemical, optical, and mechanical stimuli by changing their cell transmembrane potential (TMP). A cell's TMP is a measure of the potential difference across the cell's membrane. A TMP can be created due to different concentrations of ions on either side of the membrane. Cells typically maintain lower concentrations of ions inside the cell than the concentration of ions outside the cell to prevent the cell from swelling due to osmosis. Therefore, cells typically have a TMP or are depolarized.
A localized stimulus to an excitable cell, known as an action potential, can affect the cell's TMP. The reduction in TMP causes the cell's membrane to allow sodium ions to rush into the cell, which further reduces the cell's TMP. The reduction of the TMP is known as depolarization. A cell without TMP will swell due to osmosis, therefore, shortly after the sodium inrush the cell expels potassium through the cell membrane. Reducing the potassium concentration inside the cell decreases charge within the cell and increases the TMP. The process of restoring a cell's TMP is known as repolarization.
During the time when the cell is depolarized, it cannot be restimulated by another action potential. This interval is known as the cell's absolute refractory period. The cell's relative refractory period is the interval from partial to complete repolarization. During this time, the cell can be restimulated, but a higher stimulus is required to produce an action potential event, and the response of the excitable cell is lower in magnitude.
Nerve cells are a particular type of excitable cell that are typically characterized by a cell body from which extend dendrites and an axon. The long axon is coat in myelin sheaths and axon terminals extend from the end of the axon. When the nerve cell is stimulated, a depolarization wave travels down the axon to the axon terminals. The axon terminals respond to the depolarization wave by releasing specialized chemicals known as neurotransmitters. The neurotransmitters bind to receptors in the dendrites of adjacent nerve cells and depending on the type of receptor that is activated, will either excite or inhibit the generation of an action potential in the adjacent cell. In this way, signals are passed from one nerve cell to another and enable impulses to be carried along nerve fibers.
Neurostimulation is a term used to describe the artificial excitation or inhibiting of nerve cells. Neurostimulation is thought to be desirable as either a tool for simulating nerve function or for inhibiting the flow of information to the brain (e.g. blocking pain impulses). The ability to selectively stimulate specific nerve fibrils in a complex nerve bundle containing thousands, is a long sought capability in biomedical research. One method has been to try and stimulate the nerves using an electrode. However, placing an electrode in contact with the desired nerve fibril can be invasive. Alternatively, a nerve cuff electrode can be used. Nerve cuff electrodes typically involve placing multiple electrodes around the nerve to create an electric field designed to stimulate a specific nerve fiber.